The subject matter of the present invention relates generally to hearing aid apparatus and in particular to direct contact hearing aid apparatus mounted in the ear canal with an output transducer having its output coupling element supported to provide direct electromechanical coupling to the ossicles through the tympanic membrane. The coupling element may engage a contact element mounted on the outer surface of the tympanic membrane, and such elements may be a magnet and a magnetic member to provide a magnetic connection. The invention also relates to hearing aid apparatus employing a piezoelectric plastic film as the output transducer. An external magnetic attachment device is employed for insertion and removal of the hearing aid, radio or other electronic apparatus into the ear canal by magnetic engagement with a holder member on the housing of such apparatus. Externally actuated magnetic switch means are provided within the housing for remote mechanical switching of the electrical connections of a battery and volume control means therein.
The present invention is especially useful as a direct contact hearing aid mounted deep within the ear canal of persons who wish to conceal such hearing aid from the view of others. However, such invention is also useful for other types of external hearing aids where it is advantageous to minimize acoustic feedback phenomena.
Conventional external hearing aids, in which the input transducer is a microphone and the output transducer is a loudspeaker which modulates an air column between such speaker and the tympanic membrane of the eardrum, have several disadvantages. These disadvantages include acoustic feedback from the loudspeaker to the microphone, inefficient operation resulting in greater power dissipation and frequent battery changes, and distortion of the acoustical output due to the necessarily small diameter of the speaker diaphragm. Previous attempts to overcome acoustical feedback in conventional external hearing aids have included providing an airtight seal in the ear canal in an attempt to acoustically isolate the output transducer loudspeaker deep within the ear canal from the input transducer microphone as shown in U.S. Pat. No. 3,061,689 of McCarrell, et al., issued Oct. 30, 1962. This has been only partially successful in allowing maximal gain without feedback. This also has the disadvantage that the airtight mold used as a seal produces an uncomfortable sensation of fullness and increases the perception of internal noises such as one's own voice. In addition, in the McCarrell patent an ear mold containing the microphone, the amplifier and the battery is positioned at the external portion of the ear so that a volume control for such amplifier may be adjusted manually while the hearing aid is in place in the ear. Unfortunately, this requires that the ear mold piece of the hearing aid be located at a position where it can easily be viewed by persons talking to the wearer, which is cosmetically objectionable.
Both electromagnetic transducers and piezoelectric transducers are employed as output transducers in McCarrell, but they are not placed in direct contact with the outer surface of the tympanic membrane in the manner of the present invention. Instead, his output transducers are employed as loudspeakers to produce a sound output by vibrating a plastic diaphragm. However, in one embodiment an iron slug is mounted by adhesive directly on the outer surface of the eardrum and spaced away from the electromagnetic transducer core by an air gap whose width would inadvertently vary depending upon the position of the transducer in the ear canal. The width of such air gap is critical to efficiency since the latter varies inversely with the third power of such width. The appropriate air gap width for maximum efficiency would therefore be very difficult to obtain by positioning the hearing aid in the ear canal and to maintain with any consistency. In addition, the static undirectional stress placed on the eardrum membrane by attraction of the magnetic slug toward the electromagnetic pole piece would stress the ossicular chain and tend to weaken and tear the tympanic membrane.
The hearing aid of the present invention eliminates these disadvantages by positioning the output coupling element of the output transducer in direct contact with the outer surface of the tympanic membrane or with a contact element secured to the outer surface of such membrane, thereby eliminating any airspace between the output transducer and the tympanic membrane. As a result, there is direct electromechanical coupling from the output coupling element of the transducer to the ossicle bones through the tympanic membrane without the generation of any discernible sound waves, thereby eliminating acoustic feedback, providing a much more efficient operation and reducing distortion. Also, the lack of undirectional static stress would decrease the risk of damage to the tympanic structures.
An experimental hearing aid in which a magnet was attached by glue to the outer surface of the tympanic membrane for movement of the magnet by an induced electromagnetic field produced by a coil on the exterior ear is described by Goode, et al., in "Audition via Electromagnetic Induction" published July 1973 in Arch Otolaryngol, Volume 98, pages 23-26. This hearing aid, by employing electromagnetic induction coupling for movement of a magnet attached to the tympanic membrane, is not practical because of the large amount of power required to overcome the inefficiency caused by the gap between the magnet and the coil. Such article also describes earlier unsuccessful experiments by Wilska who attached small pieces of iron on the tympanic membrane for vibration by a coil and superimposed constant magnetic field of a permanent magnet which are placed over the ear canal, but the strong magnetic attraction apparently stretched or tore the eardrum and caused severe discomfort and pain. Wilska apparently also attached a small electromagnetic coil to the tympanic membrane with similar results except that the coil temperature also caused burning and pain. Unlike the present hearing aid, there was no direct electromechanical coupling of the output transducer of the hearing aid into engagement with the outer surface of the tympanic membrane or with a contact element provided on such membrane with all of its advantages of excellent sound fidelity, low power requirements and no pain, stress or damage to the eardrum during operation of the hearing aid.
It is interesting to note that as recently as September 1982 researchers were still attempting to implant hearing aid output transducers, such as piezoelectric ceramic vibrators, in the middle ear in order to overcome the disadvantages of conventional external hearing aids. In this regard, see the summary of Japanese research and development projects in the article "Implantable Hearing Aid Project" published in Jetro, September 1982, pages 6 to 10, which discloses a similar hearing aid to that shown in one of the embodiments of the earlier discussed U.S. Pat. No. 3,712,962 of Epley.
It has been previously proposed in U.S. Pat. No. 3,832,580 of Yamamuro, et al., issued Aug. 27, 1974, and U.S. Pat. No. 4,369,391 of Micheron issued Jan. 18, 1983, to provide a piezoelectric plastic transducer in nonhearing aid devices, such as a phonographic pick-up and pressure sensing cables, made of a piezoelectric plastic film including a natural or synthetic high molecular weight polymers and polyvinylidene fluoride. However, such piezoelectric plastic film has not previous been used in a hearing aid. It has been discovered by the present inventor that the low mechanical impedance of piezoelectric plastic film transducers closely matches that of the middle ear conducting mechanism so that it is ideal for use as the output transducer of a hearing aid. This discovery has lead the inventor to develop several different types of piezoelectric plastic film output transducers for hearing aids which are shown herein.
It is advantageous in most situations using a contact hearing aid in accordance with the present invention to have an output transducer configuration with a low mechanical impedance, because when the transducer is directly coupled with the middle ear sound transmitting system, its mechanical impedance is incorporated into that of this system. If this increase in impedance is too great, it creates an increased perception of internal sounds such as the person's own voice or chewing. This annoying effect can be minimized by using an output transducer in which the active element has high compliance and low mass. Several output transducers embodying these principles were described in my pending U.S. patent application Ser. No. 592,236, filed Mar. 22, 1984. Several further embodiments are described herein.
Bias permanent magnets have been used in electromagnetic transducer applications where they provide several advantages. They allow the use, as the active element, of a permanent magnet with a highly compliant mounting, such as a diaphragm, in conjunction with an electromagnetic coil containing a ferromagnetic core. Such ferromagnetic core greatly increases the efficiency of the coil. Wihout the insertion of a reverse bias magnet between the active magnet and the core, the magnetic field induced in the core by the active magnet would create a static force acting upon the active magnet in the direction of the core, thereby placing a static strain on the resilient mounting and decreasing its compliance. Also, this static force would greatly limit the allowable narrowness of the gap between the active magnet and the core, and such gap narrowness of the mounting is critical to the efficiency of the transducer. As the gap provided by the mounting is made smaller, the static force of the attraction will increase exponentially. Since the counterforce of the elastic mounting follows a direct arithmetic function, a point will be reached at which this counterforce will be overcome by the induced magnetic force which follows an exponential function, and the active magnet will be forced into contact with the core and will be unable to vibrate. This problem can be overcome by inserting a reverse bias magnet as a permanent magnet oriented with its polarity in the reverse direction to that of the active permanent magnet, between the active magnet and the ferromagnetic core. To limit the range of this repulsion and to create a neutral mounting point for the active magnet with a relatively narrow gap, a stronger but more distant forward bias magnet is mounted opposite the reverse bias magnet. At this neutral mounting point, there will be no static force acting upon the active magnet and, thus, maximum compliance of the elastic mounting.
Although the physical mass of the bias magnets themselves may limit the proximity of the active magnet to the ferromagnetic core, if the bias magnets are made of high permeability material they will transmit the variations in magnetic flux induced by the electromagnetic coil with an efficiency approaching that of the ferromagnetic core, so that the effective gap is essentially that existing between the active magnet and the reverse bias magnet.
To minimize the possibility that the clip arms attaching the contact element to the handle of the malleus may cause erosion of the underlying bone in contact with said arms, bioactive ceramic material such as hydroxyl apatite can be utilized in the construction of such attachment using a metal clamp with jaws lined with said bioactive material. When said bioactive material comes into contact with living bone, a slight dissolution takes place at the bone-implant interface, forming a double layer transition film of silica gel and calcium phosphate through an exchange of sodium and hydrogen ions, which crystalizes to bond the bone to the implant. In the present application, the bioactive material will act to prevent bony erosion due to static pressure or vibration at the interface between the implanted clamp jaws and the malleus. In addition, it will act to prevent instability of the mounting.